Alan R. Carroll

Stratigraphic classification of ancient lakes: Balancing tectonic and climatic controls

Lakes and lake deposits present two fundamental paradoxes: (1) Modern lakes are vastly complicated, but the rock records of lakes are relatively simple; extensive observations reveal three distinct facies associations of common and widespread occurrence. These are referred to here as fluvial-lacustrine, fluctuating profundal, and evaporative facies associations. (2) Most explanations of modern and ancient lakes attribute their nature to climate, but neither modern lake parameters (lake size, depth, and salinity) nor the character of ancient-lake strata (thickness, extent, lithology) correlate with measured or inferred climatic humidity. We propose that it is the relative balance of rates of potential accommodation (mostly tectonic) with sediment + water fill (mostly a function of climate) that controls lake occurrence, distribution, and character. Lake basins may be termed overfilled, balanced fill, or underfilled, depending on the balance between these rates. We conclude that climate and tectonics exert coequal influence on lake deposits at both mesoscales (1 m to hundreds of meters) and macroscales (hundreds to thousands of meters).

Late Paleozoic tectonic amalgamation of northwestern China: Sedimentary record of the northern Tarim, northwestern Turpan, and southern Junggar Basins

Sedimentary rocks contained in basins adjacent to the Tian Shan provide a long and complex record of the late Paleozoic continental amalgamation of northwestern China, complementing that provided by rocks preserved within the range. This record, which comprises dramatic changes in sedimentary facies, sediment dispersal patterns, sandstone provenance, and basin subsidence rates, broadly supports previous interpretations of a two-part evolution of the Tian Shan: Late Devonian to Early Carboniferous collision of the Tarim continental block with the Central Tian Shan, followed by collision of this combined block with island arcs in the north Tian Shan and Bogda Shan in Late Carboniferous–Early Permian times. The first collision resulted in widespread angular unconformities within the Tarim basin. Continued convergence following the collision created a long-lived flexural foredeep along the northern margin of the Tarim block, which received at least 2000 m of Lower Carboniferous through Lower Permian fluvial and marine sediment derived from the interior of Tarim. Subsequent Early Permian continental extension of the northern Tarim basin resulted in the deposition of interbedded nonmarine siliciclastic sedimentary rocks and mafic to felsicvolcanic rocks. Sandstone within this interval was derived from the paleo–Tian Shan, and is composed predominantly of lithic volcanic grains similar to the rhyolite. In contrast to the Tarim basin, calc-alkaline volcanic rocks and volcanogenic sedimentary rocks dominated Carboniferous and Permian sedimentation in the northern Turpan and northwestern Junggar basins. Volcanic arcs remained active in the North Tian Shan and Bogda Shan through the early Late Carboniferous, depositing a kilometers-thick interval of deep marine sediment-gravity flows in the northwestern Junggar basin. Major arc magmatism ceased in the Late Carboniferous in response to closure of the oceanic basin between the combined Tarim/Central Tian Shan block and the North Tian Shan/Bogda Shan arcs. Upper Carboniferous through Lower Permian rocks in the northwestern Junggar basin compose the sedimentary fill of a bathymetric basin of oceanic depth (on the northern side of the volcanic arcs), culminating in a 1000-m-thick marine regressive sequence. Middle to Upper Permian sandstones were derived from the uplifted paleo–Tian Shan and bear the distinctive provenance imprint of granitic rocks presently exposed within the range. Late Permian subsidence of the Junggar basin accommodated >5 km of nonmarine sediments; however, the cause of this subsidence and its relationship to regional tectonic events remain controversial.

Junggar basin, northwest China: trapped Late Paleozoic ocean

Abstract The Junggar basin originated during the late Paleozoic as either a remnant lower to mid-Paleozoic ocean basin or a mid-Carboniferous back-arc (intra- or inter-arc) basin bounded by emergent volcanic arcs south of the Siberian Craton. The retreating Junggar Sea left behind a regressive sedimentary section comprising at least 3–4 km of marine volcaniclastics in the southern Junggar area. Sandstones deposited in the Junggar basin since the Devonian are exclusively volcaniclastic, demonstrating that Precambrian basement rocks have never been exposed in the basin, and supporting the hypothesis that the Junggar is underlain by oceanic crustal materials. The mid-Carboniferous Junggar Ocean may have been a remnant ocean basin of early to mid Paleozoic age, or alternatively may have been an extending intra- or inter-arc basin formed behind an emergent volcanic arc in the northern Turpan region. Subsequent strike-slip deformation and the general sparsity of geologic and geophysical data from the Junggar area make it difficult to distinguish between these models. Late Early Permian through Triassic sediments of the Junggar basin are exclusively non-marine, deposited in a flexurally subsiding foreland basin during initial uplift of the ancestral Tian Shan mountains. The sedimentary section deposited during the Late Permian-Early Triassic appears to be inconsistent with a proposed rifting episode during this period.

Synoptic reconstruction of a major ancient lake system: Eocene Green River Formation, western United States

Numerous 40Ar/39Ar experiments on sanidine and biotite from 22 ash beds and 3 volcaniclastic sand beds from the Greater Green River, Piceance Creek, and Uinta Basins of Wyoming, Colorado, and Utah constrain ∼8 m.y. of the Eocene Epoch. Multiple analyses were conducted per sample using laser fusion and incremental heating techniques to differentiate inheritance, 40Ar loss, and 39Ar recoil. When considered in conjunction with existing radioisotopic ages and lithostratigraphy, biostratigraphy, and magnetostratigraphy, these new age determinations facilitate temporal correlation of linked Eocene lake basins in the Laramide Rocky Mountain region at a significantly increased level of precision. To compare our results to the geomagnetic polarity time scale and the regional volcanic record, the ages of Eocene magnetic anomalies C24 through C20 were recalibrated using seven 40Ar/39Ar ages. Overall, the ages obtained for this study are consistent with the isochroneity of North American land-mammal ages throughout the study area, and provide precise radioisotopic constraints on several important biostratigraphic boundaries. Applying these new ages, average sediment accumulation rates in the Greater Green River Basin, Wyoming, were approximately three times faster at the center of the basin versus its ramp-like northern margin during deposition of the underfilled Wilkins Peak Member. In contrast, sediment accumulation occurred faster at the edge of the basin during deposition of the balanced filled to overfilled Tipton and Laney Members. Sediment accumulation patterns thus reflect basin-center–focused accumulation rates when the basin was underfilled, and supply-limited accumulation when the basin was balanced filled to overfilled. Sediment accumulation in the Uinta Basin, at Indian Canyon, Utah, was relatively constant at ∼150 mm/k.y. during deposition of over 5 m.y. of both evaporative and fluctuating profundal facies, which likely reflects the basin-margin position of the measured section. The most rapid sediment accumulation for the entire system (>1 m/k.y.) occurred between 49.0 and 47.5 Ma, when volcaniclastic materials from the Absaroka and/or Challis volcanic fields entered the Green River Formation lakes from the north. Our new ages combined with existing paleomagnetic and biostratigraphic control permit the first detailed synoptic comparison of lacustrine depositional environments in all the Green River Formation basins. Coupled with previously published paleocurrent observations, our detailed correlations show that relatively freshwater lakes commonly drained into more saline downstream lakes. The overall character of Eocene lake deposits was therefore governed in part by the geomorphic evolution of drainage patterns in the surrounding Laramide landscape. Freshwater (overfilled) lakes were initially dominant (53.5–52.0 Ma), possibly related to high erosion rates of remnant Cretaceous strata on adjacent uplifts. Expansion of balanced-fill lakes first occurred in all Green River Formation basins at 52.0–51.3 Ma and again between 49.6 and 48.5 Ma. Evaporative (underfilled) lakes occurred in various basins between 51.3 and 45.1 Ma, coincident with the end of the early Eocene climatic optima and subsequent onset of global cooling defined from marine record. However, evaporite intervals in the different depocenters were deposited at different times rather than being confined to a single episode of arid climate. Evaporative terminal sinks were initially located in the Greater Green River and Piceance Creek Basins (51.3–48.9 Ma), then gradually migrated southward to the Uinta Basin (47.1–45.2 Ma). This history is likely related to progressive southward construction of the Absaroka Volcanic Province, which constituted a major topographic and thermal anomaly that contributed to a regional north to south hydrologic gradient. The Greater Green River and Piceance Creek Basins were eventually filled from north to south with Absarokaderived detritus at sedimentation rates 1–2 orders of magnitude greater than the underlying lake deposits.

Uplift, exhumation, and deformation in the Chinese Tian Shan

The terranes composing the basement of the Tian Shan were originally sutured together during two collisions in Late Devonian–Early Carboniferous and Late Carboniferous–Early Permian time. Since then, the range has repeatedly been uplifted and structurally reactivated, apparently as a result of the collision of island arcs and continental blocks with the southern margin of Asia far to the south of the range. Evidence for these deformational episodes is recorded in the sedimentary histories of the Junggar and Tarim foreland basins to the north and south of the range and by the cooling and exhumation histories of rocks in the interior of the range. Reconnaissance apatite fission-track cooling ages from the Chinese part of the range cluster in three general time periods, latest Paleozoic, late Mesozoic, and late Cenozoic. Latest Paleozoic cooling is recorded at Aksu (east of Kalpin) on the southern flank of the range, at two areas in the central Tian Shan block along the Dushanzi-Kuqa Highway, and by detrital apatites at Kuqa that retain fission-track ages of their sediment source areas. Available Ar/Ar cooling ages from the range also cluster within this time interval, with very few younger ages. These cooling ages may record exhumation and deformation caused by the second basement suturing collision between the Tarim–central Tian Shan composite block and the north Tian Shan. Apatite data from three areas record late Mesozoic cooling, at Kuqa on the southern flank of the range and at two areas in the central Tian Shan block. Sedimentary sections in the Junggar and Tarim foreland basins contain major unconformities, thick intervals of alluvial conglomerate, and increased subsidence rates between about 140 and 100 Ma. These data may reflect deformation and uplift induced by collision of the Lhasa block with the southern margin of Asia in latest Jurassic–Early Cretaceous time. Large Jurassic intermontane basins are preserved within the interior of the Tian Shan and in conjunction with the fission-track data suggest that the late Mesozoic Tian Shan was subdivided into a complex of generally east-west–trending, structurally controlled subranges and basins. Apatite data from five areas record major late Cenozoic cooling, at sites in the basin-vergent thrust belts on the northern and southern margins of the range, and along the north Tian Shan fault system in the interior of the range. The thrust belts *Now at ExxonMobile Exploration Company, P.O. Box 4778, Houston, Texas 77060, USA Dumitru, T.A., et al., 2001, Uplift, exhumation, and deformation in the Chinese Tian Shan, in Hendrix, M.S., and Davis, G.A., eds., Paleozoic and Mesozoic tectonic evolution of central Asia: From continental assembly to intracontinental deformation: Boulder, Colorado, Geological Society of America Memoir 194, p. 71–99. 72 T.A. Dumitru et al.

Lake-type controls on petroleum source rock potential in nonmarine basins

Based on numerous empirical observations of lacustrine basin strata, we propose a three-fold classification of lacustrine facies associations that accounts for the most important features of lacustrine petroleum source rocks and provides a predictive framework for exploration in nonmarine basins where lacustrine facies are incompletely delineated. (1) The fluvial-lacustrine facies association is characterized by freshwater lacustrine mudstones interbedded with fluvial-deltaic deposits, commonly including coal. Shoreline progradation dominates basin fill, resulting in the stacking of indistinctly expressed cycles up to 10 m thick. In map view, the deposits may be regionally widespread but laterally discontinuous and contain strong facies contrasts. Transported terrestrial organic matter contributes to mixed type I-III kerogens that generate waxy oil (type I kerogen is hydrogen rich and oil prone; type III kerogen is hydrogen poor and mainly gas prone). The Luman Tongue of the Green River Formation (Wyoming) and the Honyanchi Formation (Junggar basin, China) provide examples of this facies association, which is also present in the Songliao basin of northeastern China, the Central Sumatra basin, and the Cretaceous Doba/Doseo basins in west-central Africa. (2) The fluctuating profundal facies association represents a combination of progradational and aggradational basin fill and includes some of the worlds richest source rocks. Deposits are regionally extensive in map view, having relatively homogenous source facies containing oil-prone, type I kerogen. Examples include the Laney Member of the Green River Formation (Wyoming), the Lucaogou Formation (Junggar basin, China), the Bucomazi Formation (offshore west Africa), and the Lagoa Feia Formation (Campos basin, Brazil). (3) The evaporative facies association represents dominantly aggradational fill related to desiccation cycles in saline to hypersaline lakes and may include evaporite and eolianite deposits. Sublittoral organic-rich mudstone facies are relatively thin but may be (Begin page 1034) quite rich and widespread. The highest organic enrichment coincides with the deepest lake stages. Low input of land plant organic matter results in minimal lateral contrasts in organic content. In some cases a distinctive type I-S (sulfur-rich) kerogen may generate oil at thermal maturities as low as 0.45% vitrinite reflectance equivalent. Examples include the Wilkins Peak Member of the Green River Formation (Wyoming), the Jingjingzigou Formation (Junggar basin, China), the Jianghan and Qaidam basins (China), and the Blanca Lila Formation (Argentina).

40Ar/39Ar geochronology of the Eocene Green River Formation, Wyoming

The deposits of Eocene Lake Gosiute that constitute the Green River Formation of Wyoming contain numerous tuff beds that represent isochronous, correlatable stratigraphic markers. Tuff beds selected for 40Ar/39Ar analysis occur within laminated mudstone, are matrix supported, and lack evidence of reworking. These tuffs contain 2%–15% euhedral phenocrysts of quartz, plagioclase, sanidine, biotite, and minor amphibole, pyroxene, and zircon, encased in a matrix of altered glassy ash. Air abrasion and handpicking under refractive- index oils were required to obtain clean, unaltered phenocrysts of sanidine. 40Ar/39Ar age determinations from single-crystal and <1 mg multigrain aliquots of sanidine and biotite allowed the identification and exclusion of xenocrystic contamination. Laser-fusion experiments on phenocrysts from the Rife, Firehole, C Bed, Grey, Main, Sixth, and Analcite tuff beds from the Tipton, Wilkins Peak, and Laney Members yielded weighted-mean ages (±2σ analytical uncertainties) of 51.25 ± 0.31 Ma, 50.70 ± 0.14 Ma, 50.56 ± 0.26 Ma, 50.39 ± 0.13 Ma, 49.96 ± 0.08 Ma, 49.70 ± 0.10 Ma, and 48.94 ± 0.12 Ma, respectively. Ages for sanidine and biotite from the Main tuff are indistinguishable when presumably xenocrystic contaminants are excluded from the age calculation. Moreover, the 40Ar/39Ar ages are consistent with the stratigraphic order of the tuff beds and with provenance in the Absaroka and Challis volcanic fields. Our 40Ar/39Ar-based age model indicates that sediment accumulated three times more rapidly (327 ± 86 μm/yr) during the evaporative Wilkins Peak phase than the freshwater to saline Tipton (88 ± 34 μm/yr) and Laney (104 ± 18 μm/yr) phases. The much lower accumulation rates for the Tipton and Laney Members are permissive of an annual origin for <1-mm-thick laminae and precessional forcing of 1–3-m-thick depositional cycles in these units. However, previously described cycles in the Wilkins Peak Member have average durations that are significantly shorter than the 19–23 k.y. precessional modes. The Green River Formation encompasses an ∼5 m.y. period between ca. 53.5 and 48.5 Ma, spanning magnetic chrons 24n through 21r. The Green River Formation was therefore deposited during the warmest period of the Cenozoic corresponding to the early Eocene climatic optimum as defined by the global marine O isotope record. Deposition of bedded evaporites (trona) of the Wilkins Peak Member began at ca. 51 Ma, or ∼2 m.y. after the onset of the highest inferred temperatures of the climatic optimum. The Bridgerian–Wasatchian faunal turnover occurred subsequently during Wilkins Peak time, at ca. 50.6 Ma. Thus, our 40Ar/39Ar ages strongly suggest that Wilkins Peak evaporite deposition and the turnover from Wasatchian to Bridgerian fauna were not directly caused by the initiation of maximum greenhouse conditions.

Characteristics of Selected Petroleum Source Rocks, Xianjiang Uygur Autonomous Region, Northwest China (1)

The sedimentary basins of Xinjiang Uygur Autonomous Region, China, are moderately to poorly explored for petroleum. Volumetric adequacy of petroleum source rocks is a critical exploration risk in these basins, particularly because source rock data are limited. This study provides new source rock data and speculatively assesses the source rock potential of Xinjiang basins. The Junggar (Zhungaer) basin, the best explored of the Xinjiang basins and containing a giant oil field, is underlain in many areas by an Upper Permian lacustrine oil-shale sequence remarkable for its organic richness and oil source quality. Depending on its position in the basin, the Permian section ranges from immature to overmature and is inferred to be the principal source of oil in the basin. Upper Triassic-Middle Jurassic coal measures, including lacustrine rocks, constitute a secondary source rock sequence in the basin. The smaller, intermontane Turpan (Tulufan) basin contains a very similar Upper Triassic-Middle Jurassic sequence, which, where sufficiently buried, probably comprises the only significant oil source sequence in the basin. The vast Tarim (Talimu) basin offers the greatest variety of potential source rocks of all Xinjiang basins but remains the least well documented. From limited but geologically planned and focused sampling, Cambrian, Carboniferous, and Permian strata are not considered major oil contributors in the dominantly shallow marine Paleozoic section of the northern Tarim basin. Only Ordovician black shales appear to have significant potential. The Upper Triassic-Middle Jurassic sequence of the northern Tarim basin is similar to that of the Junggar and Turpan basins--a section rich in coal and lacustrine shale that constitutes another potentially significant oil source. Due to the size, stratigraphic packaging, and structural relief of the northern Tarim basin, Paleozoic and Mesozoic potential il source beds range from immature to overmature.

Permian sedimentary record of the Turpan-Hami basin and adjacent regions, northwest China: Constraints on postamalgamation tectonic evolution

The Permian marks an important, yet poorly understood, tectonic transition in the Tian Shan region of northwestern China between Devonian–Carboniferous continental amalgamation and recurrent Mesozoic–Cenozoic intracontinental orogenic reactivation. The Turpan-Hami basin accommodated up to 3000 m of sediment and is ideally positioned to provide constraints on this transition. New stratigraphic data and mapping indicate that extension dominated Early Permian tectonics in the region, whereas flexural, foreland subsidence controlled Late Permian basin evolution. Lower Permian strata in the northwestern Turpan-Hami basin consist of coarse- grained debris-flow and alluvial-fan deposits interbedded with mafic to intermediate volcanic sills and flows. In contrast, Lower Permian rocks in the north-central and northeastern Turpan-Hami basin unconformably overlie a Late Carboniferous volcanic arc sequence. These Lower Permian strata include possible shallow-marine carbonate rocks and thick volcanic and volcaniclastic rocks that are in turn overlain by littoral- to profundal-lacustrine facies. Above a regional Lower Permian/Upper Permian unconformity, regional sedimentation patterns record the development of a more integrated sedimentary basin. The Upper Permian is entirely nonmarine and can be correlated east-west along the depositional strike of the basin. The lower Upper Permian consists of a broad belt of braided fluvial deposits shed northward. These strata are overlain by fluctuating littoral- and profundal-lacustrine facies and associated fluvial facies. The uppermost Permian is characterized by shallow lake- plain and fluvial environments. The Early Permian association of diffuse volcanism and partitioning of subbasins by normal faulting is consistent with an early phase of lithospheric extension. Local relationships indicate west-east extension in the Turpan-Hami basin along faults oriented normal to Late Devonian–Carboniferous collisional sutures within the Tian Shan. The cause of extension in the wake of Carboniferous orogenesis remains enigmatic. However, the temporal and spatial relationships of the two strain regimes suggest that they are genetically related. Upper Permian stratigraphy and unconformities and local Late Permian–Triassic contractional deformation record foreland-basin development when the Turpan-Hami region became a wedge-top basin with respect to the north Tian Shan fold-and-thrust belt. Flexurally induced Late Permian subsidence is also manifested in the larger Junggar basin to the north, where >4000 m of strata are preserved in the foredeep region. The Turpan- Hami and Junggar basins were depositionally connected for much of the Late Permian when a vast lacustrine system developed across northwestern China. This lacustrine paleogeography was only occasionally interrupted, possibly by structural damming during uplift of the orogenic wedge.

Eocene clocks agree: Coeval 40Ar/39Ar, U-Pb, and astronomical ages from the Green River Formation

U-Pb ages of zircon from the Firehole and Analcite ash beds in the Eocene Green River Formation (Wyoming, United States) are indistinguishable from 40 Ar/ 39 Ar ages of sanidine after adjusting the latter to the astronomically calibrated age of 28.201 Ma for the Fish Canyon sanidine standard. Six of nine zircon analyses from the Firehole ash yield concordant ages in an overlapping cluster and give a weighted mean 238 U- 206 Pb age of 51.66 ± 0.20 Ma (full external 2σ uncertainty), indistinguishable from the recalibrated 40 Ar/ 39 Ar age of 51.40 ± 0.25 Ma (full external 2σ uncertainty), which is adjusted to eliminate samples with low radiogenic 40 Ar*. Significant 238 U- 206 Pb age scatter likely reflects some combination of minor inheritance, Pb loss, and possibly magma chamber residence. Seven of eight analyses of Analcite ash zircon yield a weighted mean 238 U- 206 Pb age of 49.23 ± 0.13 Ma (full external 2σ uncertainty), whereas the sanidine 40 Ar/ 39 Ar age is 49.24 ± 0.18 Ma (full external 2σ uncertainty). Calibrating Green River Formation 40 Ar/ 39 Ar ages to the 28.201 Ma age for Fish Canyon sanidine permits the first direct comparison of specific Green River Formation strata to the astronomical solution for Early Eocene insolation. This comparison supports the hypothesis that periods of fluvial deposition coincided with minima in long and short eccentricity, and that periods of lake expansion and evaporite deposition correspond to eccentricity maxima.